1. Field of the Invention
The present invention relates to a semiconductor device in which a voltage monitoring circuit monitors a voltage output from a voltage-decreasing regulator provided in a semiconductor integrated circuit.
2. Description of the Related Art
Conventionally, a voltage-decreasing regulator, which is provided in a semiconductor integrated circuit, supplies a power-source voltage to a plurality of internal modules in the semiconductor integrated circuit. In such semiconductor integrated circuit, there are some operational modes, such as a normal operational mode, a low-power mode, etc. Regardless of which operational mode of the semiconductor integrated circuit is selected, the conventional regulator supplies the same power-source voltage to the internal modules.
Moreover, in the semiconductor integrated circuit, the power-source wiring is provided for supplying the source voltage output from the regulator to the internal modules. However, it is not assured that the source voltage supplied through the power-source wiring in the semiconductor integrated circuit is kept at the same level. Rather, the source voltage supplied is varied depending on the wire length between the conventional regulator and the subject internal module along the power-source wiring. A relatively large drop of the source voltage supplied may occur when the wire length from the regulator is long, or when the location where the source voltage is supplied is located adjacent to the internal module which consumes a large amount of the supplied current.
Japanese Laid-Open Patent Application No. 2001-16770 discloses a power supply device which includes a voltage-increasing circuit, a detection unit, and an output-voltage control unit. In the power supply device, the voltage-increasing circuit converts the source voltage of a power supply to an output voltage and supplies the output voltage to the internal modules. The output voltage supplied by the voltage-increasing circuit includes a first step-up voltage and a second step-up voltage higher than the first step-up voltage. The detection unit detects the source voltage of the power supply. The output-voltage control unit controls the voltage-increasing circuit when the source voltage detected by the detection unit is within a predetermined range from the first step-up voltage. At this time, the voltage-increasing circuit supplies the second step-up voltage to the internal modules in accordance with the control by the output-voltage control unit.
However, the conventional power supply device of the above document No. 2001-16770 is directed to reducing the influence of a switching noise of a comparator circuit and performing accurate measurement of the source voltage supplied. The conventional power supply device serves to perform the switching operation to select one of the first step-up voltage and the second step-up voltage being supplied to the internal modules. Similar to the conventional regulator, the power supply device of the above document No. 2001-16770 supplies the same power-source voltage to the internal modules, regardless of which operational mode of the semiconductor integrated circuit is selected.
As described above, in the conventional power supply device, the output voltage supplied is fixed regardless of which operational mode of the semiconductor integrated circuit (the normal operational mode, the low-power mode, etc.) is selected. When the semiconductor integrated circuit is operating in the low-power mode, it is desired to reduce the supplied output voltage as low as possible. However, the conventional power supply device is provided to supply the fixed output voltage irrespective of the operational mode, and there is a problem in that the semiconductor integrated circuit may have excessive power consumption during the low-power mode operation.
Moreover, the degree of the voltage drop is varied with the locations within a semiconductor chip to which the source voltage is supplied. That is, the voltage drop in the power supply wiring in which a large amount of current flows is large, and, conversely, the voltage drop in the power supply wiring in which only a small amount of current flows is small.
If the voltage drop is large, the range of the operating voltage of the internal module connected to the power supply wiring will become narrow. In order to secure an adequate range of the operating voltage, it is necessary to provide the internal module with a circuit having a large drive capacity. There is a problem in that the power dissipation and the circuit scale will become large.